Well, there is a Saturn emu: BlueCrab's Yabause-DC.
Out of the current systems, the DC is the
platform for a SAT emulator. The DC's main CPU, the SH4, is somewhat backward compatible to the Saturn SH2 CPUs. They share the same instruction set. Still it's a bit more complicated than it sounds because they use a different byte order (little endian / big endian) but BlueCrab wrote somewhere on these forums that he works on a real-time byte swapper to make full use of the possibilities.
Based on Yabause v0.0.7 which as of then only emulated the Saturn BIOS, Yabause-DC v0.0.7 emulated the BIOS too. Slow, but it did. It was a plain test port without PVR acceleration or the dynarec-like byte swapping implemented. Blue Crab is still working on Yabause-DC. As with most of us, he just doesn't have as much free time as he'd wish so it will take some time until we see a new release.
There's no SEGA CD emu yet. It needs more work than e.g. a NeoGeo emulator as the SCD is more powerful than a NeoGeo. Emulating the complete MD/G hardware and the even stronger SCD on DC might be possible to a degree but needs an incredible amount of work and would implement a lot of compromises like underclocking, frameskip, probably even disabled FM music or soundFX, etc. - most-likely resulting in limited compatibility.
Could be that we'll see a working SCD emu some day, but I assume it won't ever be perfect.